There is a continuous effort to develop sensors that are lower in cost and capable of being mass produced, yet characterized by high reliability, sensitivity and linearity. Sensors finding wide acceptance on the basis of furthering these characteristics include those that utilize semiconductor materials with a micromachined sensing diaphragm, a notable example being micromachined single-crystal silicon pressure transducer cells manufactured using semiconductor fabrication processes. In the processing of such cells, a thin diaphragm is formed in a silicon wafer through preferential chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical conductivity changes with strain such that deflection of the diaphragm causes a change in resistance value of the piezoresistive elements, which can then be correlated to the magnitude of the pressure applied to the diaphragm.
Diaphragms of single-crystal silicon pressure transducer cells are typically small, rarely exceeding a few millimeters in width, and are very thin, with a thickness of often less than 100 micrometers. The use of standard single-crystal silicon wafers and standard semiconductor device fabrication processes allows many such cells to be fabricated from a single wafer, providing some economy of scale. However, silicon is susceptible to chemical attack and erosion by various media, particularly in applications where a high-pressure medium is to be sensed, e.g., automotive applications that involve sensing brake fluid, oil, coolant, transmission fluid, hydraulic fluid, fuel and steering fluid pressures at pressures of two atmospheres or more. For such applications, a pressure sensor must also be physically rugged and resistant to the hostile environment of the sensed medium, necessitating that a micromachined silicon pressure transducer cell include some form of protection in order to realize its advantageous operational characteristics in the chemically hostile environment.
Current methods for producing media-compatible sensors include enclosing a silicon sensing chip in an inert fluid, such as a silicone oil or gel, and then further separating the sensing chip from the medium to be sensed with a metal diaphragm, such that pressure must be transmitted through the metal diaphragm and fluid to the sensing chip. While achieving some of the operational advantages of silicon pressure transducer cells, the manufacturing processes for these sensors, and hence the sensors themselves, are relatively expensive and complicated. As a result, these sensors are not suitable as mass-produced sensors for automotive applications. Furthermore, the influence of the fluid in contact with the silicon pressure transducer cell over the operating temperature range and over time is of sufficient magnitude to require complex electronics to separate their effect on the silicon pressure transducer cell from the effect of pressure.
Another approach is to form a capacitor plate on a ceramic diaphragm, which is then bonded to a base with a second capacitor plate. The use of a chemically-resistant and mechanically-tough ceramic materials, such as aluminum oxide or zirconium oxide, allows the diaphragm to directly contact the medium whose pressure is to be measured, thereby eliminating the requirement for protective packaging. As the ceramic diaphragm deflects under the influence of pressure, the gap between the capacitor plates changes, causing a corresponding change in capacitance that can be correlated to the applied pressure. However, the circuit required to detect capacitance changes is somewhat complex and subject to noise corruption. In addition, attaining an adequate seal between the ceramic diaphragm and base for high pressure applications can be difficult.
Yet another approach employing a chemically-resistant ceramic diaphragm uses thick-film piezoresistors that are screen printed on the diaphragm, thereby providing for pressure sensing in the same manner as described above for single-crystal silicon pressure transducer cells. As with ceramic capacitive pressure sensors, the ceramic material is chosen to allow direct contact with the medium whose pressure is to be sensed, eliminating the need for protective packaging. While the signal detection circuitry used is less complicated than that for the capacitive sensor, the difficulty of reliably sealing the ceramic diaphragm with a base is the same as that for the capacitive sensors in high-pressure applications.
Finally, other media-compatible sensors known in the prior art employ a metal diaphragm as the sensing element. Because metal diaphragms generally deflect more for a given thickness and pressure than ceramic diaphragms, which exhibit lower elongations before breaking and are therefore designed to deflect less under pressure, sensing is performed by thin-film polysilicon resistors. The metal diaphragm is first coated with a thin-film layer of typically silicon dioxide or silicon nitride to electrically insulate the diaphragm from the thin-film resistors and their associated conductors. A thin-film polysilicon layer is then deposited on the insulating layer by chemical vapor deposition (CVD), and then etched through a mask formed by spinning a liquid photoresistive material onto the polysilicon layer, and then exposing and developing the photoresistive material to obtain the pattern required for the resistors. Thin-film conductors are then formed by evaporation.
The equipment necessary to deposit the insulating and polysilicon layers is expensive, and deposition rates are extremely slow. Deposition of the thin-film layers requires multiple patterning, exposure, developing and stripping steps for the required thin-film spun-on photoresists and metallization, and must be carried out in a controlled environment to assure that no air borne particles are present on the surface to be coated. In addition, such processes typically deposit thin-films no thicker than about 10,000 angstroms, necessitating that the surface of the metal diaphragm be extremely smooth to avoid rough surface features penetrating through or producing discontinuities in the deposited thin films.
In view of the above, metal diaphragm sensors have also been produced with thick-film metal oxide resistors. A thick-film oxide layer is formed by printing and firing a thick-film ink, after which a thick-film metal oxide layer is printed and fired on the insulating layer to form the resistors. Thick-film conductors are also typically employed with this type of sensor. A drawback with this technology is that thick-film resistors tend to have lower and less reproducible gauge factors than thin-film polysilicon resistors, and the output of such resistors can vary significantly with temperature and sheet resistivity.
From the above, it can be seen that further improvements in sensors are desired, particularly for sensors that are compatible with corrosive, high-pressure media, yet are relatively uncomplicated, low in cost, and characterized by high reliability and sensitivity.